I'm entirely certain that it is heroin, Tommy even says to Quinn that he's spending all his cash on her, and he's not smoking that much weed, on top of that, we see everyone who uses the syringes to be passed out in a lethargic state without any care in the world (Heather next to Tommy, and Quinn after she takes half the syringe and invites Tommy to take the rest), this along with Quinn's comment to think about it as "liberation" where they would give up all their worries also sounds a lot like heroin. Then there's also the case where you go get Tommy to come to breakfast with the rest of the DIKs, he is initially agitated when we meet him by his room, but by the time he comes to the table to eat he's completely mellow, no anger or anything, and he apologizes for snapping at MC, but he has absolutely no appetite, which is something that happens when you do heroin.